formaat collages wijzigen

The size of the resulting collage is 1800 x 1200 pixels and the format is a 'project' format (.pse) hiding a true .psd layered format which you can save from the menu Image > Resize.

If you open the resulting .psd in 'Expert' mode, you can not only customize your collage manually, but you also can use the 'resize' feature to define the new values, especially the width in cm and constrain proportions. Such a one meter wide print will only require a resolution of 150 ppi.

Will you lose quality in upsizing so much?

No, because each of your individual photos are 'smart' objects, they will be included from the originals of your photos and resized accordingly. On the other hand, any 'vector' data like text or shapes can be upsized without loss.

Now, the width/height ratio will stay at 18/12 (= 3/2) which will not fill the 100/80 of your project. You'll get 100 x 67 cm only.